The present invention relates to a flat cable and a flat cable producing method, in particular, to a flat cable that can be produced at low cost and that can be densely mounted.
In recent years, as various types of electronic devices that generate radio frequency signals have been developed, their use has become widespread. As a result, many electronic apparatuses are being used in offices and homes. These electronic apparatuses use coaxial cables as radio frequency signal cables.
FIG. 1 shows the structure of a conventional coaxial cable. Disposed at the center of a coaxial cable 120 is a signal line 121. Disposed around the signal line 121 is a dielectric substance 122. Disposed around the dielectric substance 122 is a ground layer 123. The outermost periphery of the coaxial cable is coated with an insulator 124. Since the cross-section of the coaxial cable 120 is circular, it cannot be flattened. Consequently, the coaxial cable 120 has a large diameter. Thus, the coaxial cable 120 cannot be densely mounted. In addition, since these layers should be cylindrically formed and deposited, a complicated production process is required. It is therefore difficult to decrease the production cost of the coaxial cable 120.
To solve the foregoing problem, Japanese patent laid-open publication No. 2001-135974 and Japanese patent laid-open publication No. HEI 11-162267 propose structures for mounting a plurality of signal lines in flat cables using liquid crystal polymer.
However, the flat cables disclosed in these Japanese patent publications cannot be suitably used for transmitting radio frequency signals. In this case, the sizes of the cross-sections of the signal lines, the thicknesses of the dielectric substances, and so forth should be adjusted so that a predetermined characteristic impedance can be obtained and insertion loss can be decreased. In addition, the ground layer should be sufficiently wider than the signal line so as to prevent signals from leaking out of the cable.
In addition, Japanese patent laid-open publication No. 2002-111233 discloses a method for forming a radio frequency transmission line on a printed circuit board. In this method, however, since the transmission lines cannot be freely bent, the method cannot be used for cables.